The present invention relates to a Ti alloy poppet valve which provides improved wear resistance and strength, and surface treatment thereof.
The largest difficulty for increasing allowable rotation speed of an engine is increase in inertial mass owing to increase in weight of valve-operating parts. If whole weight of the valve-operating parts increases, followability of a valve body to a cam decreases owing to inertial mass during high-speed rotation so as to decrease engine output performance.
Therefore, a poppet valve is molded from a low-density heat resistant Ti alloy to decrease its weight instead of a conventional heat resistant steel. However, Ti alloy has activity and is likely to adhere to another metal. Wear resistance and fatigue strength are not sufficient. Surface treatment such as nitriding and Ni plating is made on the surface of Ti alloy valve to improve wear resistance.
The nitrided valve provide high strength or hardness and wear resistance, but it is too rigid, so that it is likely to attack other parts. It is required to replace material of another valve-operating member which contacts the valve to increase manufacturing cost. A Ni plated valve does not achieve sufficient heat resistance and is not suitable as an exhaust valve.